Add a start flag, and let --check survive a first deploy

-e start=false (mode sync in CI) lands the payload and starts nothing, for
migrations. A check run on a never-deployed stack now skips Compose with a note
instead of failing on the missing directory.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ci-gitlab.md

@@ -54,6 +54,10 @@ Pipelines → Run pipeline, add a variable) and press the same deploy button. Th
 also enables the work gated behind an update run — `webapp`'s backup and
 migration.
 
+To copy the files and start nothing — the `sync` mode of the other two
+platforms — run it with `START=false` instead. `true`, `false` and empty are the
+only values accepted.
+
 `ansible:ping` is the right first thing to run after setting the variables up. It
 should print `SUCCESS => {"ping": "pong"}` for every host, and changes nothing.